1. Field of the Invention
The invention concerns a secured display on a screen of a workstation. The invention applies in particular to railroad interlock systems.
2. Description of the Prior Art
A railroad interlock system controls the track equipment of a rail network in accordance with operating requests sent out by a central station operator.
A railroad interlock system of this kind is usually associated with a supervisory system including a workstation with a screen for displaying the current status of the rail network in the form of mimic diagrams. These mimic diagrams are based on information sent back from sensors on the rail network under surveillance. An operator controls the operation of the railroad interlock system in accordance with the situation shown on the screen. Most importantly, the operator may on their own authority take decisions concerning the control of the track equipments of the rail network, in particular in the event of a malfunction of some of the equipment, with a view to guaranteeing correct operation of the network. In this context, the expression "securing the display" is to be understood as meaning detecting a malfunction of the display.
In a situation of this kind it is very important, since these are highly safety critical type operations, to be sure that the display of the mimic diagrams on the screen is an exact reflection of the current status of the rail network. In particular, it is necessary to be sure that the supervisory system is operating correctly. If it is not, it is important for the operator to be able to detect a malfunction of the supervisory system, including an error in the display of mimic diagrams. A display error can be caused by deterioration of the supervisory system, for example a program entering a loop and therefore monopolizing the central processor unit, or by faults in the memories used to store image data defining the mimic diagrams, or by a malfunction of the display screen itself (this list of possible causes of error is by no means exhaustive).